A large number of analogs of adenosine or of adenosylhomocysteine have been examined for their ability to function as inhibitors and/or substrates of adenosylhomocysteinase. The synthesis of analogs of adenosylhomocysteine by this enzyme in vivo may be used to form very potent and specific inhibitors of transmethylation reactions. The two most interesting compounds studied are 3-deazaadenosine and 3-deazaaristeromycin. Both are inhibitors of adenosylhomocysteinase, but only 3-deazaadenosine is a substrate for the enzyme. In vivo, both 3-deazaadenosine and 3-deazaaristeromycin inhibit a variety of methylation reactions, but there are differences in specificity. Both compounds exhibit antiviral activity against a number of RNA and DNA viruses. 3-Deazaadenosine, but not 3-deazaaristeromycin inhibits chemotaxis by a mouse macrophage cell line. 3-Deazaadenosine specifically inhibits synthesis of a small number of proteins, which may be due to inhibition of some reaction involved in mRNA synthesis or processing, such as methylation of the 5' cap. Differences in specificities of these compounds can be used to search for the function of specific methylation reactions in chemotaxis, RNA synthesis, and other cellular processes.